The present invention generally relates to fabricating nonvolatile semiconductor memory devices. In particular, the present invention relates to improved methods of fabricating flash memory devices with dual function bit lines.
A desire for compact size and high access speed has driven the development of EEPROM (electrically erasable programmable read only memory) flash memory devices. One such development, applicable to both conventional and SONOS (silicon-oxide-nitride-oxide-silicon) flash memory devices, provides a virtual ground array structure. While a non-virtual ground array structure has dedicated source and drain regions for reading and writing operations, a virtual ground array structure reduces the spacing between gates by employing dual purpose bit lines that can serve as either sources or drains according to the voltages applied.
Reducing the size of flash memory devices increases their speed, but speed can be enhanced in other ways. In a large array, speed is enhanced by reducing the spacing between contacts along bit lines. Contacts take up room and generally require a broader spacing between adjacent word lines wherever the contacts are to be placed. A compromise between the gains of more narrowly spaced contacts and the cost of having more contacts is to place contacts along the bit lines at every 16 word lines.
The response delay associated with polysilicon word lines can be reduced by siliciding, which provides lower electrical resistance. Siliciding is generally carried out without masking in what is referred to as a self-aligned siliciding process (saliciding). Unfortunately, saliciding has proven difficult in virtual ground arrays. The saliciding process has a tendency to cause shorting between bit lines, particularly in the absence of oxide island isolation regions.
In general, memory devices are faster and more compact than ever. However, there remains a demand for ever faster and/or more compact memory devices.
The following presents a simplified summary of the invention in order to provide a basic understanding of some of its aspects. This summary is not an extensive overview of the invention and is intended neither to identify key or critical elements of the invention nor to delineate its scope. The primary purpose of this summary is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
The present invention provides a process for saliciding word lines in a virtual ground array flash memory device without causing shorting between bit lines. According to one aspect of the invention, saliciding takes place prior to patterning one or more layers of a memory cell stack. The unpatterned layers protect the substrate between word lines from becoming salicided.
The invention provides virtual ground array flash memory devices with doped and salicided word lines, but no shorting between bit lines, even in virtual ground arrays where there are no oxide island isolation regions between word lines. Potential advantages of such structures include reduced size, reduced number of processing steps, and reduced exposure to high temperature cycling.